The Essential Oil of Hops:
Aroma and Flavor in Hops and Beerby Glenn Tinseth
Republished from BrewingTechniques' January/February 1994.

The quest for elusive hop character in beer has led researchers and brewers to break fertile ground in the discovery and control of the components that contribute flavor and aroma. A review of current research and brewing practice reveals the nature of hop aroma and guides brewers in optimizing the use of hops to achieve the desired character of finished beer.

The female flower of the hop plant (Humulus lupulus) has long been a friend to beer brewers around the world. Depending on the style of beer, hops can add balance to the finished product through bitterness, flavor, and aroma. The contribution of bitterness in the finished product through the isomerization of a-acids is well documented, but investigations into the chemistry and utilization of hop essential oils for beer flavor and aroma have, in general, failed to elucidate the complex processes involved. This article focuses on the essential oil of hops, providing background on the chemistry and composition of hop oil in hops and beer and discussing practical methods of evaluating and analyzing hops for the aromatic quality of hop essential oil. I include some methods of obtaining and protecting hop flavor and aroma in finished beer, which I think is the most important take-home message of all.

Most dedicated small-scale brewers are familiar with the wide variety of hop products available for use in brewing. The starting point for all of these is the whole, unprocessed hop flower. Hop flowers are harvested once a year (in late August and early September), processed, packaged, and held in cold storage until sold. Various levels and intensities of processing result in various end products, including Type 100 pellets (highly compressed whole cones in 1/2-oz plugs), Type 90 pellets (ground whole cones extruded into pellets), and a variety of purified and concentrated extracts and essences.

In most cases trade-offs arise when choosing among product types, especially when the brewer is concerned with hop essential oils. These trade-offs are roughly proportionate to the intensity of processing. Each product type makes a slightly different contribution to beer, providing brewers various avenues for introducing hop character into their products.

The various hop extracts and essences available give brewers even more choices of when to add hop compounds. Some British brewers have abandoned dry hopping in favor of postfermentation additions of concentrated hop oil. The hop oil extracts available today that are produced using liquid carbon dioxide are much higher in quality than the extracts of the past that were made using solvents, and they are free from the problem of solvent (hexane) contamination.

By far the most common way that hops are used is to add them to the brew kettle and boil them in sweet wort. In long boils (30-120 min), hops accomplish four primary functions: they add bitterness through the isomerization (rearrangement without change of composition) of a-acids into more stable and soluble iso-a-acids; they assist in the production of a good hot break by supplying tannins that combine with unwanted proteins; they add to beer stability by virtue of their antibacterial properties; and they lower the surface tension of the wort so that a vigorous boil can be more easily maintained.

Late kettle additions (0-30 min before knockout) are responsible for most of the "hop character" of a beer, or the hop flavor and aroma that result from the contribution of hop essential oils. Even more hop oil can be introduced into the wort by running hot wort through a bed of hops on the way to the chiller (using a hop back, or hop jack, for example). Dry hopping - adding hops to the fermentor or serving tank - also adds more hop character, though the compounds extracted by cool wort or beer are quite different from those found in wort immediately after the boil.

In a nutshell, hop character is affected both by the type of hop product you add to the wort or beer and by the method and timing of the addition. This variation is due in part to the vast array of highly reactive compounds that make up hop oil.

The aromatic and Flavor constituents of Hops

Most plant materials contain two groups of oils, differentiated by their volatility. The fixed oils, like liquid fats found in nuts, are relatively nonvolatile and contribute little to aroma. The volatile oils, also called essential oils, are so easily vaporizable that we can deduce their presence or absence simply by using our noses.

We are all familiar with perfumes and spices, both of which are loaded with either naturally occurring or synthesized essential oils. In hops, the essential oil makes up only about 0.5-3% (v/w) of the whole cone, but its contribution to beer is enormous. Consider the following: If a typical beer uses hops at a rate of 0.5 oz/gal, and we assume that the oils are 10% utilized, then the hop oils ending up in the finished beer are <0.001% (w/w) (10 ppm). Yet what is the first thing that hits the nose in a Cascade dry-hopped American pale ale? Its big floral aroma is impossible to miss. A brief examination of the chemistry of hop essential oil can explain that big impression.

Hop oils are made up primarily of a hydrocarbon fraction (which contains only hydrogen and carbon) and an oxygenated fraction (which contains hydrogen, carbon, and oxygen). Hop oils also contain small amounts of sulfur-containing compounds, but these are outside the scope of this article. The hydrocarbons typically make up 80-90% of the total oil content; the terpenes myrcene and b-pinene and the sesquiterpenes b-caryophyllene and a-humulene are the most prevalent constituents. Two of these, b-caryophyllene and a-humulene can be easily oxidized in air, thus contributing to the oxygenated fraction of the oil as well (1). Other oxygenated compounds include alcohols such as linalool and geraniol and esters such as geranyl isobutyrate and methyl dec-4-enoate (2-5). Although many brewers think that all esters are by-products of fermentation, hops can contribute a number of fruity aromas (grapefruit and pineapple, for example).

Determining "Hoppiness"

Some brewers have asked the question, "Why hasn't anyone tried to quantify the 'hoppiness' of a given hop sample like they have its bitterness (which is based on percent a-acid content)?" Because hop character is attributable to hop essential oil, one would think that the amount of oil in a hop sample would provide a means of estimating its hop aroma content. Unfortunately, the issue is not simple (6).

A hop's total oil content can indicate the overall quality of a hop sample, especially if you have a good idea what the normal oil content for that hop should be. This method may be valuable if you are buying hops from afar, sight-unseen. But because many factors affect essential oil production and preservation, every season holds the potential for significant variation in both the total amount of oil and the composition of the oil, even for a single variety. In fact, substantial differences can be detected from farm to farm, not to mention the additional variables introduced by hop processing and storage facilities. Over the period 1975- 1985, the total oil content of fresh Cascade hop samples studied at the USDA Hop Research Laboratory at Oregon State University (Corvallis) varied between 0.28 and 1.79 mL/100 g of hops; the percent myrcene, an inert component (usually the major hydrocarbon component) in the oil which has no effect on aroma potential, ranged from 46% to 82% (7). The total oil rating, therefore, does not necessarily provide useful information about the composition of the oil and thus cannot provide the information needed to evaluate the aroma quality of our hops. Three other methods, however, have proven to be quite helpful.

Just as bitterness in beer is quantified using the bittering unit (BU), Nickerson and Van Engel propose that an aroma unit (AU) be adopted. They define the AU as the sum of the 22 HACP constituents, measured in parts per million (ÁL/kg of hops or ÁL/L of wort or beer). Thus, if the AUs of a brewery's aroma hops were to change, the brewery would need only to calculate the change in the amount of hops needed to achieve the historical AUs of a given beer.

How well do AUs correlate with actual 'hoppiness'? A commercial brewery's taste panel found that hop aroma, hop taste, and dry hop aroma correlated very well with AUs (8). Using data from an earlier study (9), I checked to make sure that no more simple method was being overlooked; as a home brewer I didn't have a spare gas chromatograph laying around my basement. The data did show correlation between AUs and the total oil content in fresh hops, though the correlation was relatively weak (r = 0.449) (10), ranking only about a 3 on a scale of 1-10 (see Figure 1).

The correlation was better in hops aged for six months at room temperature (r = 0.756), but it is ill-advised for anyone to brew with hops that have been subject to such storage conditions. These data confirm the hypothesis that total oil content is not necessarily a good indicator of potential hoppiness or aroma quality. Also, I found no significant relationships between AUs and myrcene, humulene, or a-acid levels.

Low tech. For those of us without a complete lab at our disposal, a couple of tried and true low-tech (or no-tech) methods can be used for evaluating the aroma potential and quality of hops.

The first is initiated simply by grabbing a handful of hops and looking, listening, feeling, tasting, and smelling them. In reference to evaluating fine aroma hops, Jean De Clerk wrote, (11)

"Aroma is tested by smelling a crushed handful of whole cones. The hops should have a pronounced aromatic smell free from extraneous taints and odors, [list of unpleasant odors]. . . . Smell may also be tested by rubbing the cones between the fingers, which splits the lupulin grains. The aroma of the sample should not be sharp, but fine and mellow."

Although this sort of manual evaluation of hops will give you a good feel for the overall quality of the hop and its potential dry hop aroma, it provides little quantitative information. Most of what you will smell is the hydrocarbon fraction of the essential oil, and this fraction is rapidly lost during the boil or changed during fermentation and is, in fact, rarely found in beer (9). If after crushing the hops in your hands you pause for a minute or two to let the most volatile of the hydrocarbons dissipate, you will be better able to evaluate the aroma quality (6).

The second method is to make a hop tea by boiling or steeping a hop sample in water. The preparation process evaporates a significant portion of the hydrocarbons and gives you a better impression of how a particular sample will perform in the brew kettle. To be as consistent as possible, I recommend always making the tea in the same container (a 1-L Erlenmeyer flask is good) and using the same water-to-hop ratio (2 g hops/600 mL water) (12). Prepare the tea the same way you plan to use the hops in your beer, boiling (or steeping) for a predetermined time. After cooling the tea, add water to bring the volume back to your starting point and evaluate the flavor and aroma both for quality and quantity/intensity. Although this method uses no fancy instrumentation, it can be very effective, especially if good sampling notes are kept. More information about using hop teas to evaluate hop aroma will appear in a future issue of BrewingTechniques. Regardless of which low-tech method you use, if it smells bad, don't use it.

Hop essential oils, like hop bitter resins, are easily lost during storage due to their susceptibility to oxidation. Workers performing storage trials at the USDA Hop Research Laboratory determined that oil losses ranged from 28% to 90% after six months at room temperature, depending on the variety (9; see also article beginning on page 26 of this issue). These losses can be slowed by storing hops in the freezer, preferably in a package that allows no air or water exchange ("barrier packaging"). The best advice is to buy the best hops you can find and take good care of them, especially if they are aroma hops.

As mentioned above, the compounds found in hop oil are not all the same as those found in hopped beer. As many brewers already know, late kettle additions contribute different qualities than do steeped hops and dry hops. Fermentation and finishing processes also affect hop character. Consequently, a brewer needs to find the type of hop character desired and then be consistent in the method used to achieve it.

POSTFERMENTATION LOSSES

Once you have achieved the Holy Grail of hoppiness, and the bottle is capped or the keg is sealed, hop character begins its inevitable downhill slide. According to Peacock and Deinzer, the two main ways in which hop oil components in finished beer can be lost are migration into the packaging material and chemical degradation. They write (2),

"The major mechanism for the loss of hop aroma compounds from beer appears to be chemical degradation. The most likely pathways for degradation of terpenoid and sesquiterpenoid compounds are reaction with oxygen in the headspace of the bottle and acid hydrolysis."

They go on to say that bottle cap liners tend to absorb many hop compounds, especially hydrocarbons and longer chain ketones and esters. Hop oil alcohols appear not to be absorbed well by cap liners. Because we can do nothing about acid hydrolysis, this diagnosis points to a rigorous elimination of oxygen from all beer packages and to the use of a nonabsorptive bottle cap liner. Oxygen scavenger caps are available (PureSeal, Zapata Technologies, Hazleton, Pennsylvania), but their performance with respect to absorption of hop oil compounds has not yet been tested.

Hop Aroma Made Simple

Four important points simplify dealing with hop essential oils. First, start with the best hops you can get and store them in the freezer in barrier packaging if possible. Second, monitor the aroma quality of your hops using either the aroma unit (if you can) or by physical evaluation. Third, be consistent in your brewing process, making adjustments based on your evaluations. Fourth, try to eliminate as much oxygen as possible from your packaging process. The last two points will yield benefits in other areas, increasing the pay-off of added care in these areas.

Hop oil research is a rapidly changing field, and new findings pop up every year. The consensus is that the marvelous hop aroma and flavor we all enjoy is probably due to a synergistic combination of hop oil compounds and their oxidation and fermentation products, some of which may not even have been discovered yet. One thing is certain: even though I may not know exactly what I am tasting in my dry-hopped IPA, I like it!